1. Technical Field
The present invention relates to valves, and in particular, to proportional valves in which the output flow is proportional to the pressure drop across the valve.
2. Description of the Related Art
Modern gas turbines used in the generation are very large and capable of a continuous power output between 200-500 megawatts. Such high output requires significant fuel consumption on the order of 200-400 gallons per minute. Water can be injected into the combustion chambers to reduce emissions from the turbine. Water injection also serves to increase maximum power output of the turbine by increasing mass flow through the turbine. Water flow is injected at flow rates comparable to the fuel flow rates.
Given the high flow requirements of large industrial turbine systems, the proper operation of the metering valves used to control water injection is critical. Failure of the metering valve can lead to the turbine tripping off-line. Tripping off-line can be harmful to the moving elements of the turbine and leaves the turbine inoperable for lengthy periods while the problem is addressed and the turbine is brought back on-line. Power output of the plant is diminished during the entire period the turbine is off-line.
Problems are encountered with existing metering valves with respect to valve stability, leakage and particle contamination, among other things. The high flow rate environment in which the valve operates gives rise to high flow forces acting on the metering member of the valve. These flow forces acting on the metering member can cause disruptions or slight position changes of the metering edge at the interface with the metering orifice(s). This “chatter” in turn leads fluctuations in the metered flow and thereby transient or sustained instability. The high flow rates and flow forces can also cause seal failure resulting in valve leakage. Since the valve is used to inject water into the combustion chamber of the turbine, which is typically at elevated pressures, valve leakage can permit back pressure to restrict or back up flow from the water system. Valve operation can also be adversely affected by the build up of particles in the flow path. Close tolerances and sliding metal-to-metal contact can cause either the metering member or the seat, or both, to wear and introduce particles into the internal areas of the valve. In addition to causing galling, these particles can impair movement of the metering member or disturb the metering edge/orifice interface. All of these things impact valve operation and can lead to untimely turbine shut-down and thereby loss of plant power output.
Accordingly, an improved valve is needed for use in water injection into the combustion areas of gas turbines, particularly large industrial turbines used for power generation.